1. Field of the Invention
This invention relates generally to display devices, and more particularly to a system and method of dynamically correcting display characteristics.
2. Description of the Background Art
Display devices, including cathode ray tube (CRT) monitors, generally function in a raster configuration. In a raster display, individual picture elements (pixels) of the displayed image are represented by spots illuminated in order from left to right by a horizontal scan which progresses incrementally and vertically down the screen after each horizontal line is finished. In a CRT monitor the spots are small phosphor dots illuminated by a sweeping cathode ray of electrons. One challenge in making and using display devices is keeping characteristics of the display, such as brightness, hue, and convergence, uniform throughout the screen.
Referring now to FIG. 1, a spatial representation of an electron gun and CRT screen is shown. The CRT screen 100 contains a layer of glass 102 coated with a phosphor layer 104. A shadow mask 106 collimates the electron beam 110 emitted by electron gun 112 so that it falls on phosphor layer 104 at those places representing the image color of the respective beam. Typically phosphor layer 104 contains red, green, and blue spots, which may be illuminated by individual electron guns. The electron beam 110 traverses the CRT screen 100 from left to right, as indicated by the arrow, causing the illuminated spot 114 to progress from left to right. However this motion is not visible to the eye because of the eye""s response time.
Due to imperfections in manufacturing, phosphor layer 104 is not uniform in thickness or responsiveness throughout the CRT screen 100. Similarly shadow mask 106 is not at a uniform distance from phosphor layer 104. For these and other reasons, spot parameters of the CRT display, such as brightness, hue, convergence, and beam landing, vary throughout the CRT display screen. In order to compensate for this lack of uniformity, the circuits driving the electron guns have traditionally allowed for calibration adjustment by a technician.
Having a calibration technician individually adjust each individual characteristic""s uniformity for every monitor manufactured becomes an expensive undertaking. Furthermore, monitors lose their uniformity due to changes in temperature and ambient magnetic fields, and also due to aging, among other factors. Monitors increasingly are used in situations such as aboard aircraft where the environment changes rapidly throughout the day. Therefore, there exists a need for an improved system and method for dynamically correcting the display characteristics requiring only minimal and infrequent calibration by a technician.
The present invention includes a system and method for dynamically correcting display characteristics to compensate for non-uniformities arising from many causes, including both manufacture and subsequent aging and environmental changes. In the preferred embodiment, the display screen is divided logically into tiles, and then the initial correction parameters of the characteristics under consideration are measured at the vertices of each tile. These initial correction parameters are stored in non-volatile memory for later use by the dynamic correction circuitry.
The dynamic correction circuitry automatically synchronizes the correction waveforms, which are functions of the locations on the physical display screen, with the control signals of the displayed image. The horizontal size is synchronized by an analog phase-locked loop, and the vertical size is synchronized by a line compensation circuit. The position of the image is synchronized by a pair of similarly-designed digital phase-locked loops. Once the correction signals are synchronized with the control signals of the displayed image, there exists a fixed one-to-one correspondence between logical picture elements (pixels) of the displayed image and the correction signal values corresponding to the physical phosphor spots on the display screen.
Due to the one-to-one correspondence between the logical pixels of the displayed image and physical phosphor spots on the display screen, the previously stored initial correction parameters of the characteristics under consideration may be used to dynamically correct for uniformity of these characteristics. The dynamic correction circuitry may use the stored correction parameters, referenced to the vertices of the tiles, to modulate the gain control voltage of the CRT video pre-amplifier, whose signals are referenced to the logical pixels of the displayed image.
The values at one of the vertices of each tile could be used without variation throughout the tile for correction of display characteristics. However this would yield a discontinuous correction function. If applied to the gain control voltages, such a discontinuous correction function would create tile-sized regions with discontinuous display characteristics at the edges of the tiles. Such discontinuities would be both noticeable and objectionable to the viewer of the display. To prevent this, the preferred embodiment of the present invention performs linear vertical and horizontal interpolation on the correction values between the vertices of the tiles.
When the CRT electron beam first enters a tile, the dynamic correction circuitry retrieves the correction values of the four vertices, and then determines the slope of the lines between the correction values at the vertices down the two vertical sides of the tiles. Knowing these slopes, the initial value at the uppermost vertices, and the distance down from the vertices, a linear interpolation value at any point on the two vertical edges of the tiles may be produced. Each time the electron beam enters the tile from the left, the two linear interpolation values at the beam""s intersection with the two vertical edges of the tile is determined. Then, using these two interpolated values, the dynamic correction circuitry performs horizontal interpolation along the path of the beam. In this manner, across the complete row of tiles traversed by the electron beam, a continuous linear interpolated display correction function is derived and sent to modulate the gain control of the amplifiers driving the electron guns, or directly to drive the deflection.